The present invention relates to a method for connecting electronic components to a carrier substrate an arrangement for connecting electronic components to a carrier substrate and a method for examining a connection between electronic components and a carrier substrate.
In is conventional to equip a carrier substrate with electronic components in a flip-chip process or ball grid array (BGA) process. In these methods, the electronic components are provided on their connection side with a plurality of solder xe2x80x9cbumpsxe2x80x9d or xe2x80x9cballsxe2x80x9d and are then placed, connection side down, on a carrier substrate provided with contact surfaces mating is being effected in that the contact surfaces, or xe2x80x9cpads,xe2x80x9d corresponding to solder bumps are alignedly assigned. The solder bumps used in the flip-chip process are usually about 75 to 80 xcexcm in diameter and those used in the BGA process are usually about 500 to 700 xcexcm in diameter. The carrier substrate is, for example, a ceramic substrate, a printed circuit board, a silicon substrate or the like. The solder bumps are then soldered to the pads of the carrier substrate in a reflow soldering process in which the solder bumps are melted in a reflow furnace and wet the contact surfaces of the carrier substrate.
Such a method is described, for example, in PCT application No. WO 98/14995, U.S. Pat. No. 5,284,796 and U.S. Pat. No. 5,246,880. During the flip-chip process, a plurality of electrically conductive connections that corresponds to the number of pads to be contacted are made simultaneously between the pads of the electronic component and the carrier substrate.
Because of the arrangement of the connecting contacts that are produced between the electronic component and the carrier substrate during reflow soldering, visual inspection is impossible. To be able to perform an inspection of the connecting contacts, in particular to check to make sure that the melted solder bumps have wet the contact,:surfaces of the pads on the carrier substrate, it is conventional to subject the bonding arrangement, consisting of the electronic component and the carrier substrate, to x-radiation and to analyze a prepared radiograph. Depending on the material used for the solder bumps, it is possible to achieve contrast visualization on the radiograph that show the solder bumps and the regions of the composite surrounding them. Depending on the resolution of the x-ray apparatus used, missing solder bumps or bridging between adjacent solder joints can readily be detected by this means. However, nonexistent or only partial wetting by the solder bumps of the contact surfaces of the pads on the carrier substrate, for example due to contamination of the pads, is not possible. These so-called xe2x80x9ccold solder jointsxe2x80x9d hinder or prevent the operation of the electronic components, and it is therefore imperative that they be detected in a quality check.
The method according to the present invention offers the advantage that nondestructive examination of electrically conductive connections made by a flip-chip or BGA technique can be performed in a simple manner. Due to the fact that at least one solder bump is deformed in the bonding plane during soldering to achieve a degree of deformation that permits the analysis of said degree of deformation by a radiograph of the connection site, not only the presence of a solder joint, but also its proper wetting of the pad to be contacted can be checked via the intensity variation of the x-radiation passing through the bonding arrangement or by a two-dimensional or three-dimensional radiograph of the connection site.
In an advantageous embodiment of the present invention, particularly for use with the flip-chip technique, the solder bumps undergo a distribution of their material during the soldering, such that their thickness decreases continuously toward the margin, the distribution of material, for example is determined by a solder stop mask that encompasses the pads of the carrier substrate. It is thereby advantageously achieved that, the initial size,and thus the initial mass of the solder bumps being known, the solder bumps can undergo a defined deformation within the bonding plane. Depending on the arrangement of the solder stop mask, this results in a distribution of material that decreases toward the margins of the solder bumps, so that a defined deformation of the solder bumps takes place. On subsequent x-irradiation of the connection site, the x-rays are absorbed to different degrees by the material of the solder bump, according to the distribution of the material of the solder bumps that has occurred, thus giving rise to an intensity variation in which the x-rays passing through the bonding arrangement exhibit a continuous transition from a maximum intensity to a intensity and vice-versa. This, continuous transition between the minimum intensity and the maximum intensity provides a simple means of detecting wetting of the contact surface of the pad. Particularly if the diameters of masking openings in the solder stop mask are selected for a solder bump diameter within defined ranges, a defined distribution of the material of the solder bump can be achieved during the reflow soldering of the components on the carrier substrate. This therefore produces the continuous variation of the thickness of the solder bump viewed in the bonding plane, and thus the continuous transition between a minimum and a maximum intensity of the x-rays passing through the bonding arrangement.
The method according to the present invention for examining a connection between electronic components and a carrier substrate further permits, in a simple manner and with high precision, the quality assessment of contact points obtained by a flip-chip process or the BGA technique. Because an influence on an intensity variation of x-rays passing through the bonding arrangement is analyzed in a region of transition from a soldered solder bump to the region surrounding it or on a two-dimensional,or three-dimensional radiograph of the connection site, the solder bumps being deformed during soldering in such a way that when the pads are properly wetted it is possible to measure a continuous transition in the intensity variation or a visible deformation of the solder bump on the radiograph, defect-free or defective contact points can be recognized from the radiographs obtained.
Due to their deformation during soldering, the solder bumps undergo a distribution of their material in which the volume (thickness) decreases toward their margins, causing a continuous transition in the intensity of the measured x-rays. Since x-radiation that is applied uniformly within the bonding plane to the bonding arrangement obtained is absorbed or transmitted differently, according to the distribution of the material of the solder bumps. This is what produces the intensity variation on the radiograph. If the solder bumps are not properly wetted by the pads, the intended distribution of the material of the solder bumps does not take place, and there is, therefore, no measurable corresponding continuous transition of the intensity distribution of the x-rays. Such unwetted or insufficiently wetted solder bumps are distinguished by an abrupt transition of the intensity, distribution. It can therefore be concluded from the abrupt variation in intensity that a xe2x80x9ccold solder jointxe2x80x9d is present. A nondestructive and precise analysis can be performed in this manner, particularly in the case of the relatively small solder bumps used in the flip-chip technique.
The unequivocal deformation of the kind that can be obtained in particular with the relatively large solder bumps used in BGA techniques can be rendered visible, and therefore made susceptible to analysis on a two-dimensional or three-dimensional radiograph. Due to the relatively large volume of the solder bumps, a continuous transition of the intensity variation cannot be detected in this case. Here, the deformationxe2x80x94with an abrupt transition in intensity between the solder bumps and the region surrounding them, evincing flawless wetting of the padxe2x80x94is clearly recognizable on the radiograph.
In a further advantageous embodiment of the present invention, the pad of the carrier substrate is encompassed by a solder stop mask the opening of which is larger than the pad. This advantageously makes it possible for deformation of the solder bump to take place during the soldering of the bonding arrangement so that edge faces of the pad that extend substantially perpendicularly to the bonding plane of the bonding arrangement can be co-wetted by the material of the solder bump. Because the solder stop mask is spaced away from the pad, this gap can be utilized to permit deformation of the material of the solder bump within the gap, the edges of the pad simultaneously being wetted when proper wetting occurs.
The proper wetting of the edges of the pad can be checked by an advantageous method for examining the connection between the electronic component and the carrier substrate. By the preparation and analysis of a three-dimensional radiograph of the bonding arrangement in the region of a layer that lies in a plane with the at least one pad of the carrier substrate, proper wetting of the edges can be demonstrated in a simple manner on the radiograph of the layer. Because only the layer in which the pads are disposed is picked out from the bonding arrangement as a whole and visualized, the presence of material deformed into the plane of the pad during the soldering, so that said material can wet the edges, can be identified by a ring-shaped pattern on the radiograph.
Moreover, in an advantageous embodiment of the present invention, wetting of the edges of the pad can be checked by a two-dimensional radiograph of the bonding arrangement. In a two-dimensional radiograph, wetting of the edges can be detected very advantageously in that the intensity variation of the x-ray beams passing through the bonding arrangement exhibits a characteristic saddle shape, which, in a simple manner, furnishes evidence of proper wetting of the edges.
Also, an advantageous embodiment of the present invention, the deformation of thexe2x80x94essentially roundxe2x80x94solder bump can be accomplished by defined shaping of the pads. During the soldering, the solder bump wets thee shaped pad and thereby essentially assumes its shape. Defined shapes for the pad are, for example, oval, triangular polygonal shapes or the like.
Wetting conforming to the shape of the pad produces a set deformation of the solder bump that can be detected on a two-dimensional radiograph. If the shape of the solder bump matches the known shape of the pad, it can be assumed that complete and therefore proper wetting of the pad has taken place. If the shape of the solder bump on the radiograph matches, for example, the original shape of the solder bump, particularly a round shape, it can be inferred from this failure of the solder bump to assume the shape of the pad that improper wetting of the pad has taken place.